1. Field of the Invention
The present invention relates to an optical switch and a matrix optical switch, and particularly to an optical switch which switches optical paths of a light signal propagating through a channel waveguide and a matrix optical switch in which multiple of the optical switches are arranged in a matrix.
2. Description of the Related Art
Optical communication networks are developing, from: point-to-point optical communication, in which nodes are connected individually; through optical communication, in which Add-Drop Multiplexing is performed between points; and further to optical communication in which plural nodes are directly connected, without converting a light signal into an electric signal. Therefore, development of various optical components necessary for the above optical communication becomes important, such as optical splitter/couplers, optical multiplexers, optical demultiplexers, optical switches, and the like. Among these, matrix optical switches are some of the most important components, being used for switching light signal paths among plural optical fibers in response to demand, or for switching light signal paths in order to secure diversion paths in the case of a network failure.
The optical switches include a bulk type of optical switches in which prisms, mirrors, fibers, and the like are mechanically moved to switch the light signal paths, and optical waveguide types of optical switch. The bulk type of optical switch has the advantage that wavelength dependence is small and loss is relatively low. However, there are various problems with the bulk type of optical switch such as: low switching speed; unsuitability for formation into matrices, due to the difficulty of miniaturization; unsuitability for mass production, because the assembly and adjustment process is complicated; expense; and the like. On the other hand, because the optical waveguide type of optical switch is significantly superior to the bulk type of optical switch in terms of switching speed, miniaturization, integration, mass production, and the like, the optical waveguide type of optical switch is being avidly investigated.
Optical waveguide type of matrix optical switches can be divided into two main modes. In the first mode, the paths of propagating light signals are switched by connecting a branching type of channel waveguide between input and output ports, and optical switches or optical gates, operated by predetermined principles are arranged at branching points. In the second mode, a light deflector is provided between the input and output ports to deflect the incident light from input ports toward output ports.
Currently, the matrix optical switch of the first mode is being most actively investigated, because of its design flexibility and small optical loss. Generally in the first mode of matrix optical switch, a channel waveguide is formed in a thin film made of LiNbO3, compound semiconductor, quartz, polymer, or the like. At crossover portions on each path there is provided either: an optical switch electrically controlling the direction of travel of the light; or an optical gate, electrically controlling the direction of travel of the light by opening and closing.
The operating principles of the optical switch include: a method of controlling the light signal path by applying an electric field to a directional coupler in which two optical waveguides are arranged close to each other; a Mach-Zehnder type of method in which an input light beam is separated into two light beams by a directional coupler, phase difference is provided between the light beams passing through the respective paths by means of a refractive index generated by an electric field, and output ends are switched by controlling interference states using a directional coupler positioned on an exit side; a method of switching light signal paths by controlling interference between optical modes at X-crossover portions; a so-called digital type of method in which light signal paths are switched by controlling a field distribution in transverse direction of the optical mode, by means of a refractive index generated by an electric field at Y-branching portions or at asymmetrical X-crossover portions; and a method of switching light signal paths in which total reflection or Bragg reflection is made to occur by providing electrodes at X-crossover portions to control the refractive index (Japanese Patent Laid-Open (JP-A) No. 7-318986 and Japanese Patent Publication (JP-B) No. 6-5350).
Among the above, the digital type of optical switch is superior in operational tolerance. In the digital type of optical switch, after light signal paths are switched with a predetermined voltage or current, this state can be maintained, and plural operation points are not generated, even if a voltage or current greater than predetermined is applied thereto. Further, advantages such as a digital type of optical switch independent of the wave polarization being possible, small degree of wavelength dependence, and the like, make the digital type of optical switch particularly noteworthy among optical switches.
However, in the conventional digital type optical switch, when compared with other types of optical switches, there are the problems of increased drive voltage (or increased drive current) and increased electrode length.
FIG. 18 shows a standard Y-branching type of structure for a digital type of optical switch. In the optical switch having the structure shown in FIG. 18, electrodes 2 constituting an optical control portion are provided at the branching portion of a Y-branching type of channel waveguide 1. An acute angle portion of the crossover portion of the channel waveguide 1 has a shape with a crossing angle less than 1° in which the channel separation gradually narrows to become zero. Because of this, with a patterning process of photolithography, it is difficult to produce an ideal shape due to resolution limitations. Therefore, usually it is necessary to form a shape where the tip end is not sharp and the distance between the channels is not less than 1.5 μm, as in the acute angle portion 3 depicted in FIG. 19. The shift from the ideal shape greatly affects the degree of loss and crosstalk, because the optical control portion is located at the branching portion on the downstream side of the crossover portion in the light propagation direction.
For example, with an open angle of the Y-branching of 0.5° and the refractive index of one of the branched waveguides being decreased by about 0.0008 due to the electro-optic effect, as long as the acute angle portion has the ideal shape shown in FIG. 18, crosstalk can be decreased. In other words the difference in light quantity between outgoing ports can be made greater or equal to 20 dB when light from an incident port is guided to the outgoing ports or other. On the other hand, if the shape is not sharp, as shown in FIG. 19, the difference in light quantity between the outgoing ports is degraded to about 12 dB. In order to increase the difference in light quantity between the outgoing ports to 20 dB, a larger change in refractive index is required. That is, in a Y-branching type of digital optical switch, there is a problem that a drive voltage or drive current increases.
Further, because an electrode is formed on the channel waveguide having a width of a few micrometers, production errors during photolithography easily occur, and symmetry of switching characteristics is easily lost.
In an X-crossover type of total reflection optical switch, as shown in FIG. 20, digital type of operation can also be performed. An X-crossover type of total reflection optical switch is suitable for high-speed response because electrode length can be more easily shortened when compared with other types. In addition, because the optical control portion is located within the crossover portion, unlike in a Y-branching type of optical switch, the X-crossover type of total reflection optical switch is less sensitive to the above-described production limitations. In the total reflection optical switch, an incident light beam 4 propagates rectilinearly when the refractive index of a channel waveguide 1 is uniform. When a voltage is applied to an electrode 2 to decrease the refractive index of a reflection plane 5 to the refractive index necessary for total reflection, the incident light beam 4 is totally reflected in the reflection plane 5. A crossing angle 6 of the channel waveguide 1 and an angle formed by the incident light beam 4 and the reflection plane 5 (reflection supplementary angle 7) are determined by the degree of decrease in refractive index of the reflection plane 5. The degree of decrease in refractive index becomes smaller, i.e., a drive voltage or drive current is lowered, as the crossing angle 6 and the reflection supplementary angle 7 are decreased. The crossing angle 6 can usually be decreased to about 0.5°.
However, in reality, the crossing angle of the X-shaped crossover portion is in the range of about 1° to about 2°, and there is a problem that drive voltage increases or crosstalk increases.
For example, an X-crossover type of total reflection optical switch in which a channel waveguide having a width of 4 μm with a crossing angle of 1.0° is formed by diffusing Ti into LiNbO3 is described in C. S. Tsai, et al., J. Quantum Electronics, (1978) 513. In the total reflection optical switch, it is expected that a response speed of 5.9 GHz can be obtained by providing electrodes with a gap of 4 μm while a taper type channel waveguide having a maximum width of 40 μm is provided in order to decrease crosstalk at the crossover portion. However, the drive voltage becomes as large as 50V.
The X-crossover type of total reflection optical switch in which an epitaxial PLZT thin film waveguide layer is grown on a sapphire substrate, which is an insulating material, to form a channel waveguide having a crossing angle of 2.0° and a width of 20 μm is described in K. Wasa, et al., J. Lightwave Technology, (1984) 710. In the total reflection optical switch, the X-crossover type of total reflection optical switch is formed by providing electrodes with a gap of 4 μm on the channel waveguide, and a response speed of 1 GHz is obtained at 4.7V. However, crosstalk is still as large as 12 dB.
An X-crossover type of total reflection optical switch having a crossing angle of 4.0° and a width of 14 μm, which is formed by using a polymer waveguide, is described in T. Ichigi et al., OFC 2002, 187. In the total reflection optical switch, crosstalk is decreased such that the difference in light quantity between outgoing ports is 30 dB or greater. However, drive electric power as large as 100 mW is required because the thermo-optic effect is utilized, and the response speed is only about 1 ms. That is, when the crossing angle of the X-crossover portion is relatively large, while crosstalk is decreased, electric power consumption is increased. In an optical switch made of polymer, since the thermo-optic effect is utilized, the merit of a high-speed response of the total reflection optical switch cannot be utilized.
As described above, in a total reflection optical switch operated by controlling the refractive index, a digital type of response can be obtained, and the total reflection optical switch is suitable for a high-speed response. However, with the total reflection type it is difficult to obtain an optical switch in which both the drive voltage or drive current is low and crosstalk is low.